Explanation

An escalator is a motorized stairway. Black Hat installs a chin-up bar at waist height. Chin-up bars are typically capable of holding up a 300 pound (130 kg) person without moving. Being at waist height makes it hard to duck under, and hard to hurdle. Either way, on a crowded escalator during a high traffic period, avoiding the bar will be extremely difficult. The first people would probably stumble backward to avoid it, which would collide them into the passengers behind them, creating a domino effect all the way down. Of course, as the escalator continues to move people forward, but the wave of falling people is moving backward, its apparent speed will be slower than it actually is.

That is the joke in the title text, that over two hours the wave can reach the bottom only three times, whereas if this were a simple staircase the event would only last once and be over as quickly as people can fall down.
The people that actually made it to the bottom were unable tue the emergency shutdown because Black Hat disabled it.

Transcript

[Black Hat is standing on an escalator as it ascends. He is carrying a pole with what looks like a bracket on each end, resting on his shoulder. In front of him is Ponytail, and in front of her is a punk with spiked hair and pimples. Behind Black Hat is Cueball. Behind Cueball is a man wearing glasses with a goatee standing next to someone with short hair.]

[The view closes on Black Hat and Cueball. In the background a girl can be seen standing on the descending escalator.]

Cueball: This is a long escalator.

Black Hat: 70 meters. Longest in the country.

[In the background the girl from the last panel has now passed the group and a few other people can be seen descending.]

Cueball: Why're you carrying a chin-up bar?

Black Hat: Why aren't you wearing a hat?

[The view opens up a bit more to show the two riders ahead, and the two behind.]

Cueball: I'm not really a hat person.

Black Hat: And I'm not really a Not-carrying-a-chin-up-bar person.

[Close up on Cueball]

[The view opens up to show the same people in the first panel. They're near the top of the escalator now and Ponytail is beginning to step off.]

Cueball: Seriously, why did you bring it?

Black Hat: How should I know? I'm not a psychologist.

[As Black Hat steps off the escalator he turns and installs the chin-up bar such that it blocks people from leaving the escalator, about waist height. Cueball turns to observe what Black Hat is doing.]

Twist

Click click

[They get onto the descending escalator. The man with glasses and a goatee and his companion are blocked from leaving the escalator by the chin-up bar.]

[The next panel is large.]

[The view shows an extended section of the escalator, the top right has become a pile of people all squished together and on top of each other. One person has grabbed another by the hair and is standing on a third person in an attempt to not fall. Someone is falling off the pile and another person is running down the escalator to avoid them. People closer to the bottom of the escalator are looking horrified at the scene ahead of them. In the background hat man and his companion are visible. Black Hat is looking toward the bottom of the escalator, not caring or noticing the chaos unfolding. Cueball looks back pensively.]

Trivia

It would appear that the goatee'd man behind Cueball has been seen before as the Psychologist from 435: Purity. It would also seem that the person he is with might be the Sociologist from that same comic.

If it is the psychologist, he should have been able to diagnose why Black Hat was carrying a Chin-up bar.

Discussion

I know the Wheaton Metro station and its escalator very well (I live about 5 miles from there). There are several issues with the comic.

There isn't a convenient place that the panels showing diagonal motion (1-6 & 10) can be shown from. The entire escalator set is in a solid tube through the rock. I guess that it could be done from the *third* escalator.(See http://en.wikipedia.org/wiki/Wheaton_(WMATA_station) ) (This would be with (viewed from the bottom, the center going up, the left going down and the right either not moving or going up)

There are emergency cut off boxes both at the top and the bottom of the escalators.

There is a kiosk about 40 feet from the top of the escalator staffed by an employee next to the fare gates.

In panel 9 there are two raised disks between the up and down escalators. While this is true in some of the other escalators, the ones for Wheaton are very close together and there is only one column of raised disks.Naraht (talk) 16:41, 28 February 2013 (UTC)

- What can we learn from this? - That not all of the facts in XKCD comics are right, and some are outright exaggerated (thank you Mr. Naraht). I personally learned that life will keep hurting me on purpose until I figure out how to help myself (Thank you Life and thank you Mr. XKCD for this lesson). - E-inspired (talk) 16:46, 28 February 2013 (UTC)

This is a comic. It's Fiction. It does not claim to be the "Wheaton Metro" escalator anyway - that was in the commentary. Relax. 74.213.186.41 19:16, 22 March 2013 (UTC)

While I agree that Naraht needs to calm down (I used to live 1 mile from the Wheaton Metro, and would walk up it as fast as I could), you cannot say that the comic did not claim to be the "Wheaton Metro." Black Hat describes it as 70m long, and the longest in the country. There is only one escalator that fits that description, and it is in Wheaton, Maryland. It's like seeing them going up an elevator, never seeing what they are ascending, and having one of the people mention that they are going to the top of the tallest free-standing stone structure in the world, 555 feet, 5 1/8 inches. There is but one structure that fits that description: the Washington Monument. A description can make a claim without using the exact wording. 108.28.72.186 03:44, 7 August 2013 (UTC)

But this is still fiction, and it is completely OK to make changes to fit the need of the comic. Also drawing someone from a position where you could not put a camera... really? I mean have you any idea how they film TV-series, where there are only two walls in the set? This is exactly the same. This is way to show us what happens, without taking a strange view from the top or where else it would be realistic to have a real camera... ;-) --Kynde (talk) 04:22, 7 August 2016 (UTC)

Grammatically, the last sentence should be "The people that actually made it to the bottom were unable to use the emergency shutdown because Black Hat had disabled it. ", (adding 'had' to 'disabled') using the past perfect for the last clause because it refers to an earlier time than the time of trying to use the emergency shutdown, even earlier than the events of the first panel. -CFitz 108.162.231.221 10:39, 12 November 2013 (UTC)

The title text does not say "those few who reached the bottom." It says "those few who escaped." Several people could have gotten around the chin-up bar or jumped the divider to get on the down-escalator. The explanation of the title text does not take this into consideration.108.162.219.202 20:20, 3 January 2014 (UTC)

The last two comments have now been taken into consideration in the explanation. --Kynde (talk) 04:24, 7 August 2016 (UTC)

This is, in my opinion, one of the most horrifying xkcd comics. I remember talks about a similar incident in the Moscow subway, when something was blocking the exit from the escalator. Some people, trying to save themselves from the stampede, climbed on top of barriers between the escalators, which were not designed to support such weight, and fell to their death in the bowels of the giant machine. 162.158.182.28 09:54, 5 September 2017 (UTC)

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